Switching from silicon to carbon has not been possible because technologists believed they needed graphene material in the same form as the silicon used to make chips: a single crystal of material eight or 12-inches wide. The largest single-crystal graphene sheets made to date have been no wider than a couple millimeters, not big enough for a single chip. Chou and researchers in his lab realized that a big graphene wafer is not necessary, as long they could place small crystals of graphene only in the active areas of the chip. They developed a novel method to achieve this goal and demonstrated it by making high-performance working graphene transistors.
“Our approach is to completely abandon the classical methods that industry has been using for silicon integrated circuits,” Chou said.
In their new method, the researchers make a special stamp consisting of an array of tiny flat-topped pillars, each one-tenth of a millimeter wide. They press the pillars against a block of graphite (pure carbon), cutting thin carbon sheets, which stick to the pillars. The stamp is then removed, peeling away a few atomic layers of graphene. Finally, the stamp is aligned with and pressed against a larger wafer, leaving the patches of graphene precisely where transistors will be built.
The technique is like printing, Chou said. By repeating the process and using variously shaped stamps (the researchers also made strips instead of round pillars), all the active areas for transistors are covered with single crystals of graphene.
“Previously, scientists have been able to peel graphene sheets from graphite blocks, but they had no control over the size and location of the pieces when placing them on a surface,” Chou said.
One innovation that made the technique possible was to coat the stamp with a special material that sticks to carbon when it is cold and releases when it is warm, allowing the same stamp to pick up and release the graphene.
Chou’s lab took the next step and built transistors — tiny on-off switches — on their printed graphene crystals. Their transistors displayed high performance; they were more than 10 times faster than silicon transistors in moving “electronic holes” — a key measure of speed.
The new technology could find almost immediate use in radio electronics, such as cell phones and other wireless devices that require high power output, Chou said. Depending on the level of interest from industry, the technique could be applied to wireless communication devices within a few years, Chou predicted.
“What we have done is shown that this approach is possible; the next step is to scale it up,” Chou said.
This is critical because indium is used now and is getting very expensive and we are running out.
As a critical component of optoelectronic devices, transparent conductive coatings pervade modern technology. The most widely used standard coating is indium tin oxide (ITO), used in nearly all flat panel displays and microdisplays. Causing problems for manufacturers, though, Indium is expensive and scarce and demand is increasing. From the depressed levels of $60/kg in 2002, indium prices rose to over $1,000/kg during the summer of this year. Recently, prices have fallen back to between $400-$500/kg. But, geologists say the cost of indium may not matter soon, because the earth’s supply of this element could be gone within just a few years.
“The graphene-based solar cells show a relatively lower efficiency than metal oxide coating based solar cells” he says. “There is still large room for improvement of the device performance by, for example, further increasing the conductivity of the graphene films via the use of large graphene sheets with lateral dimensions on the micrometer scale, or exploitation of a new and special structure of solar cells based on graphene window electrodes. Of course, we also face a number of challenges going forward, such as perfection of the graphene structure; a balance of conductivity and transparency of the graphene film; fabrication techniques for graphene-based optoelectronics, just to name a few. Nevertheless, we believe this is a new starting point in the direction of graphene window electrodes, which may open a door for further development of novel optoelectronics.”